In this study, Regional Rainfall Frequency analysis has been carried out based on L-moment in the Brahmaputra Valley. For this purpose, the monthly maximum rainfall data of the Pre-monsoon (March-May), Monsoon (June-September), and Post-monsoon (October-December) seasons for 11 sites (data varying from 14 years to 23 years) have been considered for our study. The heterogeneity measures for three seasons of the study region are calculated. The five probability distributions namely, generalized extreme value (GEV), generalized logistic (GLO), generalized Pareto (GPA), generalized normal (GNO), and the Pearson Type III (PE3) have been considered. The Z-Statistics criteria and L-moment ratio diagram for goodness of fit tests have been used for identifying the best fitting distributions for each season of the study area. The GLO is identified as the best fitting distribution for the Pre-monsoon, and Monsoon season while PE3 distribution for the Post-monsoon seasons of the study region. The regional relationships for each season of the study regions are developed. For assessment of identified probability distributions for various return periods a Monte Carlo Simulation procedure along with Root Means square error (RMSE) and Bias have been used